Apples : You can use any variety of apple you like and have on hand. I will often use a mixture of Granny Smith and Gala apples, but I also like Fuji and Honeycrisps, among others.
Toppings : I usually sprinkle mini chocolate chips and a few Heath bits on top. You could also use granola, shredded coconut, caramel, slivered almonds, raisins, etc.
To make ahead of time : Toss the sliced apples in a little bit of lemon juice before assembling the nachos to keep them from browning. Keep in refrigerator for up to a few hours, until ready to use. If you need to store them longer (or have leftovers...but we never have), they will keep for 3-4 days in a covered container.